Adrenal regeneration hypertension (ARH) is a form of normokalemic, low renin experimental hypertension in the rat induced by damaging the adrenal gland. It is an animal model of human low renin essential hypertension. Abundant evidence suggests a mineralocorticoid etiology, but the exact culprit has not been identified. We have isolated and purified a steroid from the urine of castrated rats with ARH utilizing a mineralocorticoid radioreceptor assay. Of all urinary steroids with mineralocorticoid radioreceptor activity, this steroid is excreted in the largest amount. We have chromatographic evidence of the existence of this steroid in the urine of a patient with low renin, spironolactone responsive hypertension who does not have increased production of any known mineralocorticoid, thus suggesting a role for this steroid in human hypertension. We propose to identify the structure of this steroid, synthesize it, develop a radioimmunoassay, study its mineralocorticoid and hypertensinogenic properties, and study its regulation and metabolism. Measurement of palsma and urinary concentrations and production rate of this steroid throughout the course of ARH will be determined in conjunction with similar determinations for the known mineralocorticoids. Studies to determine the site of action of this steroid will further define the pathogenesis of ARH. The identification and study of this steroid is of utmost importance in the understanding of ARH and by inference will provide data relevant to the pathogenesis of some types of human low renin hypertension.